我是初学者，我正在尝试学习如何在Android上使用API。我正在做比特币价格应用程序来学习API。但我有一些问题。我在mainactivity.xml中创建了一个进度条和一个价格TextView。默认情况下，我将progressbar设置为visible而textview不可见。我认为非常简单，我创建了一个新的AsyncTask类，在后台完成后，我将设置进度条不可见，并在onPostExecute（）中显示textview以显示价格。但它没有用。进度条一直在旋转，它没有显示价格，我不明白它为什么不起作用。
MainActivity
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.TextView;
import org.json.JSONObject;
import org.json.JSONTokener;
import java.net.URL;
import java.net.HttpURLConnection;
import java.io.InputStreamReader;
import java.io.BufferedReader;
public class MainActivity extends AppCompatActivity {
public static String line;
public static ProgressBar progressBar;
static String API_URL="https://api.coindesk.com/v1/bpi/currentprice/try.json";
static TextView priceTextView;
static TextView timeTextView;
static int priceInt;
@Override
protected void onCreate( Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
priceTextView=(TextView)findViewById(R.id.priceText);
timeTextView=(TextView)findViewById(R.id.timeText);
progressBar=(ProgressBar)findViewById(R.id.progressBar);
retrievePriceTask retrievePriceTask=new retrievePriceTask();
retrievePriceTask.execute();
}
}
class retrievePriceTask extends AsyncTask <Void,Void,String>{
@Override
protected void onPreExecute() {
MainActivity.progressBar.setVisibility(View.VISIBLE);
MainActivity.priceTextView.setVisibility(View.INVISIBLE);
}
@Override
protected String doInBackground(Void... urls) {
try {
URL url = new URL(MainActivity.API_URL);
HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
try {
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(urlConnection.getInputStream()));
StringBuilder stringBuilder = new StringBuilder();
while ((MainActivity.line = bufferedReader.readLine()) != null) {
stringBuilder.append(MainActivity.line).append("\n");
}
bufferedReader.close();
return stringBuilder.toString();
}
finally{
urlConnection.disconnect();
}
}
catch(Exception e) {
Log.e("ERROR", e.getMessage(), e);
return null;
}
}
@Override
protected void onPostExecute(String s) {
MainActivity.priceTextView.setVisibility(View.VISIBLE);
MainActivity.progressBar.setVisibility(View.INVISIBLE);
try{
JSONObject jsonObject=(JSONObject)new JSONTokener(MainActivity.line).nextValue();
MainActivity.priceInt=jsonObject.getJSONObject("bpi").getJSONObject("TRY").getInt("rate");
MainActivity.priceTextView.setText(MainActivity.priceInt);
}
catch (Exception e){
e.printStackTrace();
}
}
}
我的Logcat
07-16 10:20:08.884 2269-2269/zaaa.trinki E/AndroidRuntime: FATAL EXCEPTION: main
Process: zaaa.trinki, PID: 2269
java.lang.RuntimeException: Unable to start activity ComponentInfo{zaaa.trinki/zaaa.trinki.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference
at zaaa.trinki.retrievePriceTask.onPreExecute(MainActivity.java:94)
at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:587)
at android.os.AsyncTask.execute(AsyncTask.java:535)
at zaaa.trinki.MainActivity.onCreate(MainActivity.java:55)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
07-16 10:37:42.062 16262-16262/zaaa.trinki E/AndroidRuntime: FATAL EXCEPTION: main
Process: zaaa.trinki, PID: 16262
java.lang.RuntimeException: Unable to start activity ComponentInfo{zaaa.trinki/zaaa.trinki.MainActivity}: android.content.res.Resources$NotFoundException: String resource ID #0x0
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: android.content.res.Resources$NotFoundException: String resource ID #0x0
at android.content.res.Resources.getText(Resources.java:274)
at android.support.v7.widget.ResourcesWrapper.getText(ResourcesWrapper.java:52)
at android.widget.TextView.setText(TextView.java:4122)
at zaaa.trinki.MainActivity.onCreate(MainActivity.java:63)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
答案 0 :(得分：0)
使用ProgressDialog代替ProgressBar：
ProgressDialog myProgressDialog = ProgressDialog.show(Context CONTEXT, String TITLE, String MESSAGE, boolean INDETERMINATE, boolean CANCELABLE);
myProgressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
并在你的onPostExecute（）中放入myProgressDialog.dismiss（）